1. Field of the Invention
The invention relates to a method for improving the magnetic properties, particularly intrinsic coercivity, of particles of a permanent magnet alloy of a rare earth element, iron and boron composition.
2. Description of the Prior Art
Permanent magnets of a neodymium, iron, boron composition (Nd-Fe-B), which are well known in the art, are produced by practices including sintering, hot deformation or plastic bonding. The sintered magnets and hot deformed magnets are generally used in applications requiring relatively high magnet properties, particularly energy product, while the bonded magnets are used for applications requiring a moderately high energy product with the shape of the magnet being complex. Bonded magnets comprise particles of a permanent magnet alloy dispersed in a matrix of nonmagnetic material.
In the production of bonded magnets, isotropic Nd-Fe-B permanent magnet alloy powder is produced by the rapid quenching of molten alloy by the well known practice of melt-spinning, and the subsequent heat treatment of the melt-spun alloy ribbons to achieve high coercivity. Melt-spun ribbons of permanent magnet alloy are difficult to process into bonded magnets because particles resulting therefrom are of flat, plate-like shape, e.g. flakes. These flakes are crushed to produce fine powders to facilitate the use thereof in forming bonded magnets. Although these crushed materials have found commercial success in producing bonded magnets, they are nevertheless difficult to process in the conventional injection-molding equipment used to produce plastic bonded magnets because of the relatively poor flowability of the crushed powder.
It is known to produce permanent magnet alloys in powder form by the use of gas atomization. Gas-atomized particles are characterized by a spherical shape. It is also known to produce particles of alloys of this type by casting the alloy and then crushing the solidified casting to produce particles. These particles, are of angular configuration. Both the angular and spherical powders are of a shape that is suitable for use in producing bonded magnets. The spherical shape is preferred for this purpose because the flowability thereof is superior to angular-shaped powder. In this regard, gas-atomized powder typically has a particle size range of 1 to 300 microns.
It has been determined, however, that as-atomized Nd-Fe-B powder has intrinsic coercivity (H.sub.ci) levels too low for use in the production of bonded magnets. Consequently, attempts have been made to increase the intrinsic coercivity of the as-atomized powder by heat treatment, alloy modification, particle size control, and combinations of these factors. Prior to the present invention, a practice has not been available to uniformly achieve the intrinsic coercivity values in gas-atomized powder rendering the same suitable for use in the manufacture of bonded permanent magnets. This has also been the case with the angular powder resulting from pulverizing of a casting of the permanent magnet alloy. Hence, bonded magnets wherein the alloy particles are dispersed and bonded in a non-magnetic matrix material of a plastic composition, have been commercially produced only from particles resulting from melt-spun ribbon of the Nd-Fe-B permanent magnet alloy.